May 9, 1878] 



NATURE 



47 



To work this apparatus it is necessary to take off the 

 liquefying tube T and all the pieces of the upper part ; 

 also the lateral screw e' and its tube A' ; then, after 

 having screwed on to the joint R the piece N, which 

 serves as a stop-valve, the mercury should be turned dry 

 and quite pure, into the wrought-iron reservoir B, up to 

 the level of the edges N n'. The sides of this iron reser- 

 voir are very resistant and are able to support strong 

 pressures. 



The tube t having been filled with gas for liquefaction, 



it is gently forced into the mercury of the reservoir B ; 

 the part N being taken out, the mercury which flows out 

 is collected. When the tube A rests on the leather of 

 the bottom of the length of the reservoir, the screw e' 

 is re-screwed very tightly. The apparatus is inclined a 

 little to get rid of the excess of mercury, in order that its 

 level may remain below the lateral hole by which the 

 pressure is introduced. 



The support S with refrigerating envelope M is then 

 re-screwed on the upper part of the ajutage A ; it rests- 



Au T 1 execution 



upon leather. The safety-bell-jar c is movable; it is 

 intended to stop the pieces of glass should the tube T be 

 broken. The stop-cock r lets the water flow from the 

 envelope M. The lateral screw with the tube a' is re- 

 fixed, to which is soldered the small metallic tube T v, by 

 which pressure is introduced. 



The hydraulic pump, which Ducretet and Co. have 

 constructed specially for this apparatus, is for the pur- 

 pose of compressing water about the mercury contained 

 in the reservoir. The two valves E e' may be introduced 

 by the orifices closed by the screws E e' ; the valves may 

 be thus tested and easily changed without undoing any 



part. The reservoir of water R is placed outside ; it is 

 then easily seen to and kept full. 



Before setting the hydraulic pump to work, we with- 

 draw as completely as possible the screw-plunger piston 

 V by moving the fly-wheel. The action of the lever E 

 enables us to obtain easily a pressure of 200 atmospheres. 

 This pressure may then be increased by the gentle intro- 

 duction of the plunger-piston v. The liquefying tube T 

 is of thick glass ; it has a resistance of about 400 atmo- 

 spheres, but it is better not to exceed a pressure of 300 

 atmospheres. The second screw v' is intended to pro- 

 duce expansion. 



NOTES 

 One of the most effective methods of acquiring a headache is 

 a good round of sightseeing, especially in a museum, collection, 

 or picture-gallery ; it is quite a comfort to get among a collection 

 of any kind, the sight or catalogue of which does not make one 

 ill by anticipation. Happily the headachy feature is generally 

 absent from the collection of objects exhibited at the Royal 

 Society conversazioni, and in this respect and because of its great 

 interest, the collection brought together last Wednesday week 

 was quite a model. Prof. Snellen's two modes of testing for 

 colomr-blindness ought to have been the first thing looked at, 

 because then the guests would have been in a position to estimate 

 the value of their observations. The inspection caused much 



amusement, and in some cases astonishment. The "Mechani- 

 cal Chameleon," to exhibit the mixture of two colours in any 

 proportion, was interesting, as was also Woodward's new 

 rectangular prism illuminator, to be used with immersion lenses. 

 The President's photographs of scenes and objects in the Rocky 

 Mountains were specially attractive. Other objects which attracted 

 considerable attention were — A large Holtz electric machine (by 

 Ladd) consisting of twelve rotary and twelve stationary plates,, 

 thirty inches diameter, exhibited by Mr. W. Spottiswoode, 

 Treas.R.S. A microspectroscope with improvements,— (i> 

 quick movement of the slide carrying the slit; (2) scale for 

 registering position of slit; .(3) arrangement for comparing 

 three spectra, and for splitting a single spectra ; (4) new form of 



